The present invention relates to a device intended to attenuate or even eliminate the unwanted movements to which weather balloons are subject during ascent. The device is more particularly intended for balloons of the closed type in which the envelope is made of an elastic material and which serves to-measure wind in the atmosphere, i.e. to measure wind speed and direction. To perform such measurements, balloons are tracked either by a global positioning system (GPS) transponder integrated in a radiosonde secured beneath the balloon, or else by tracking radar. In order to enable wind speed to be measured under good conditions, it is necessary for the balloon to follow as accurately as possible the movements of the atmosphere, which is not true of conventional balloons. When a sonde is suspended beneath a normal weather balloon of substantially spherical shape, it is subjected during the ascent stage to oscillations of very large amplitude. These oscillations are created by unwanted movements of the balloon itself, and if accurate wind speed measurements are desired, it is essential to minimize the amplitude of the unwanted movements of the balloon throughout the ascent stage.
An object of the present invention is to improve the quality of wind measurements made during the ascent of weather balloons presenting an inflatable envelope fitted with at least one GPS type sonde. According to the invention, this object is achieved by the balloon being provided with a device that comprises a spoiler secured to a portion of the wall of said balloon that is situated in the top hemisphere of said balloon during ascent, in such a manner that said spoiler transmits any aerodynamic force acting thereon to said balloon and said spoiler modifies the air streams following the outside surface of said balloon so as to prevent said air streams from adhering to the surface of the bottom hemisphere of said balloon during ascent.
The device of the invention can significantly increase the stability of a balloon during its ascent, the device being more specifically adapted to weather balloons of the closed type having an elastic envelope. It is well known that during the ascent of such balloons, air flow conditions can be supercritical, which means that air streams adhere in laminar manner to the wall over a large fraction of the bottom hemisphere of the balloon. These conditions give rise to instabilities in the movement of the balloon, and consequently to poor tracking of the wind by the balloon. It is therefore essential to ensure that conditions are no longer supercritical and thus that air streams separate from the wall of the balloon before reaching the bottom hemisphere. The object of the present invention is to ensure that such separation takes place so as to obtain good measurements of wind speed.
Thus, according to a first characteristic of the invention, air streams are caused to separate from the top hemisphere of the balloon during ascent by means of a single spoiler fixed rigidly to the wall of the leading (top) hemisphere of the balloon, with the size of the separation spoiler being selected to be large enough to generate a turbulent wake which is capable of spreading by degrees to adjacent streams, thereby disturbing the flow over very large portions of the balloon.
In a preferred embodiment, said spoiler is constituted by a plate-forming element having sharp edges and having a mean plane that intersects the surface defined by said balloon. In order to fix said spoiler or said element, the device advantageously further comprises a system including a spar suitable for connecting said spoiler to said wall portion of said balloon, said spar being secured to said envelope of said balloon.
Thus, the spar is suitable for being fixed to the balloon and said spoiler is suitable for fixing to the spar. In order to fix the spar to the balloon, it is preferable for said spar to present an internal shoulder formed by an external clamp and for the system to include an elastically deformable part inside said balloon suitable for being engaged inside said spar so as to bear against said internal shoulder and pinch said envelope between said part and said spar, thereby fixing said spar to said portion of the wall of said balloon.
It will be understood that the spar must be deformable so as to enable the clamp to be placed around said spar and form the internal shoulder.
In practice, the elastically deformable piece and the non-inflated balloon portion corresponding to the wall portion situated in the top hemisphere are inserted together into the spar which is initially tubular in shape. Thereafter, the bottom portion of the spar is clamped using the clamp so as to form an internal shoulder on which the elastically deformable piece is suitable for bearing so as to hold said balloon portion against the shoulder. Thus, once the balloon has been inflated, the spar is secured to the balloon portion.
Advantageously, said plate-forming element is suitable for being fixed in adjustable manner on said spar, thereby enabling it to be given a position relative to the balloon that minimizes disturbance.
In a preferred embodiment of the invention, said plate-forming element presents cylindrical type curvature with its concave side being directed upstream relative to said air streams. Thus, the concave side of the plate-forming element constitutes an obstacle to air while the balloon is in the ascent stage.
In another preferred embodiment of the invention, said plate-forming element is pierced by orifices in such a manner as to create grating turbulence suitable for preventing said air streams from adhering to the surface of the bottom hemisphere of said balloon during ascent.
In another preferred embodiment of the invention, said plate-forming element presents two portions having mean planes that form an angle of less than 180xc2x0.
In this embodiment, the spoiler comprises two plane faces facing each other at an angle which can be selected to have arbitrary value, said surfaces optionally being of different dimensions so that under the effect of forces due to the speed of ascent a torque is created tending to cause the balloon to turn, thereby causing the turbulent disturbance of the wake to propagate laterally in more effective manner.
Advantageously, in order to achieve the same result, said plate-forming element is fixed in said wall portion of said balloon in such a manner that the mean plane of said element is inclined relative to the horizontal so as to cause said balloon to rotate about its vertical axis during ascent.
Preferably, said plate-forming element presents a longitudinal slot.
Thus, in accordance with these characteristics of the invention, said spoiler is constituted by one or more plane or concave surfaces which are used for disturbing air flow over the top portion of the balloon during its ascent, said surfaces being held rigidly secured to the wall of the balloon. Thus, the aerodynamic forces applied to said spoiler are transferred to the balloon and modify the way it moves.
In order to ensure that said spoiler does indeed transmit the forces to which it is subjected, it is rigid relative to the aerodynamic stresses. However, advantageously, said spoiler is elastically deformable and is made of a flexible material so as to be capable of deforming without damaging the envelope of the balloon in the event of coming into contact therewith on launching.